Pressure responsive valve positioner



Feb 1954 J. P. HENDERSON ETAL 2,667,860

PRESSURE RESPONSIVE VALVE POSITIONER Filed April 26, 1952 2 Sheets-Sheet1 FIG! INVENTORS: JAMES P. HENDERSON BY JQHN GALL ATT'YS 1954 J. P.HENDERSON ET AL 2,667,860

PRESSURE RESPONSIVE VALVE POSITIONER Filed April 26, 1952 2 Sheets-Sheet2 FIG.4 5

' INVENTORS: JAMES P. HENDERSON JOHN G. GALL BYIM M 5 ATT'YS PatentedFeb. 2, 1 954 PRESSURE RESPONSIVE VALVE rosrrronna James P. Henderson,La Grange Park, and John C. Gall, Chicago, 111., assignors to McAlearManufacturing Company, Chicago, 111., a corporation of IllinoisApplication April 26, 1952, Serial No. 284,674

This invention relates to a'positioning apparatus and more particularlyto an apparatus for making small changes in the position of anautomatically controlled element such as a valve disc, sluice gate,louvre, damper, aileron, etc., with great accuracy.

Various types of apparatus are known for positioning valves and otherdevices in response to the action of a spring-loaded bellows on apivoted lever. The lever is repositioned by the action ofcompensating'levers. Considerable leverage is required and friction inthe lever system limits the accuracy of positioning.

In a type of device used for positioning a valve the compound leversystem of the above described apparatus is replaced by a compressionspring to absorb motion of the main'valve stem and apply a reactivethrust proportional to the movement of the stem. Because of the phenomenon of hysteresis, the s'p'ring does not resume the same lengthafter reduction of the compressive load that it occupied before'conrpression. The accuracy of positioning thevalve disc is thereby limitedin this type of device.

It is an object of thisinvention to pr'ovi de'a reliable, accurate,rapidly'operating positioning apparatus which is'substantially free fromfrictional effects and hysteretic'losses.

Another object of the inven'tionis to provide a sensitive, ruggedposition'er which is actuated and controlled by fluid-pressure.

A further object of the invention is to provide an actuating device ofthe piston or' diaphragm type which is movable with great accuracy inresponse to a change in fiuid pressure.

Still another object of the invention is to provide a valve positionerhaving a'syst'em of di aphragms which regulate the "flow of a fluidunder pressure to a piston or diaphragm.

a Another-object cr me invention" istoprov'ic I a positioner iin which areloala'r'niai'ng follow' up is effected b 'ca'm means": f Anotherobject of the invention 'i s t o provide a positioner in which arebal'a'ncing 'follow-up-i's' eifectecl by a cam'means which means isreadily adjustable to compensate for departure from the specified springrate of a ran'g'e'spring' Another object of the invention is to providea rebalancing cam'means which'is readily adiustable to fit thestroke'offa' valve without changing t' 5: A still iurther'obiect 'of theinvention is" to provide means for adjusting the rate of response of thepositioning device 1 v The foregoing and numerous" ot important 4Claims. (01. 121-41) '2 objects, advantages and inherent functions ofthe invention will become apparent as the same is more fully understoodfrom the following description, which, taken in connection with theaccompanying drawings, discloses a preferred embodiment of theinvention.

In the drawings:

Figure 1 is a vertical sectional view of one embodiment of the valvepositioning apparatus according to this invention;

Figure 2 is an enlarged sectional view of the double disced'valveillustrated generally in Figure 1;

Figure 3 is a top plan view of the apparatus shown in Figure 1;

Figure 4 is a vertical sectional view of the posi tioner applied to aspring return diaphragm operated valve; and

Figure 5 is a fragmentary sectional view similar to Figure abutillustrating another embodiment of the cam arrangement of the invention.

In accomplishing the foregoing objects and in accordance with thepresent invention there is now provided an apparatus for positioning aremote member rapidly and accurately. A particular feature of theinvention is a cam means operatively associated with a positioningapparatus to controlit by restoring the position of a portion ,of theapparatus in response to stroking of the remote member., The apparatusis operable in response to controlled fluid pressure and comprises inaddition to the cam means a fluid pressure operated divider or diaphragmmeans operatively associated with a member to be positioned, guide meansassociated with said divider or diaphragm means and movable axiallywithin a casing, 'means on one side of said divider or diaphragm meansfor constantly'urging said first means in one direction, means forsupplying pressuratothe'oppositeside of the divider -o rdiaphragmmeans-in response to the output of a fluid pressure controller,This last-named means is a plural diaphragm .unit. v

The invention further provides means for ad- -ng and limiting the rateof flow of fluid pressure between the chambers of the plural diaphragmunit, thereby controlling and speeding up the rate of change of theactuating fluid pressure applicable to the diaphragm or divider meansandthereby providing a more positive and rapid response to changes incontroller output pressure. 7 v p j Referringtothe drawingsin detail thevalve positioning apparatus I is shown in a preferred arrangement inwhich it is located above a valve 2, the disc 3 of which is to bepositioned. The tip 4 of the valve disc 3 is adapted to seat in a valveseat 5 and thereby to control the flow of fluid passing between theinlet 6 and the outlet I. The disc 3 has attached thereto a valve stem 8which reciprocates withina body 9 "and :is-surrounded by the upperandlowerpacking glands .I 0 and I I. A spring I2 is provided whichencircles the stem 8 and bears against the packing glands I0 and I I,thereby forcing them to contact the stem 8 in,

fluid-tight engagement. A weep liole-*9a'projec.ts through the body 9.

The stem 8 extends into tthe a cylinder through the threaded opening435111 the end portion I4 thereof. The endzl5r of .the stem fl. is

adapted for threaded engagement with a threaded opening in thehorizontal .dividen .I 6.- of a piston H. The vertical portion" I8represents the circumference of the piston I! and is-adap'ted to engagethe annular casing 20. Annular packing .-rings =2 I and :2 Z move with--thepiston J I. and maintain :fiuid-t-ighte engagement Vbetweentheguide: and the casing :20. .Awindow.-:or- :slit .I9 is provided in-*casingw2 Ii. to. receive: the head of. a screw 13. The :head -ofthesscrew -13.- also. serves as an index to indicatelthe positionof-.the--.valve disc.

The diaphragm-containingabody '23 is engaged with the; top of. the.wall. .2 0 and .-isi adaptedato contain a plurality ofediaphragms.The.-body..23 is made up of. a: plurality of spacer discs interposedebetween- .the diaphragms .and.adapted. to be fastened together by meansof the long bolts 80 .(E-ig. 3) Thesseveral uiiaphragms are .moreconveniently grouped into a lowerdiaphnagmassembly-rand an upper;diaphragm assembly. The lower assembly comprises-.the.diaphragm..24..and the diaphragm 25. An annu1ar-..space.-26- isformed-between the diaphragms 24.and.25 and is-adapted to receive .asupply of; air. at regulated pressure through :the opening 21.. which.is connected to .a regulated-air supply :28. "The. chamber- .2 9-..extends .from .the underside of. .thed-iaphragm down to the. divider. 1Sin :the piston l7.

.Annpper diaphragm-assembly having. a lower portion comprising. a-.small vlower diaphragm. .30 and. a. larger upper. diaphragm 3 I..in-spaced relationship is located abovethe diaphragm .2 Sand isseparated. therefrom. by... the. spring 8 Icand the annular space" 32....'Ihe ,space .32..is. connected. by meansof .the conduit. .33. to. alarger conduit 34 Which-.connectswith. the-.space 2.9. J'Ihe adiaphragmsand: 3 I .are. separated. by. an annular space-3'5 whichis. connectedthrough .theport 36 to a-sourceot-air. under .variable-pressure. 31,.asfrom a controlling. apparatus .(notshown) Thelupper. diaphragm.assembly. also having an upper. portion. comprisingalarge .diaphragm 38and. a. smaller diaphragm .39 in spaced relation- .ship....is.lo.cated..above .the-..-.diaphragm.:- 3 In and spaced therefrom bytheannulanspacedfl. This space 40 exhausts .to .the. atmospherethrough .theport 4| inthe body. 23. ;The.upper..assembly is normally; urgedvdownwardly. by. .means of L .the spring 142. .The spring is locatedintheheadfl of the body. .23 and.is.heldin..positionbymeans of .theirecessedsplug- 44 whichrreceives the adjusting screw.45. .-.A.-cap-46is adapted tofit on the head .43..and .to protect. the screw .fromaccidental ,morement. The annulan-space 41;,exists between thediaphragms 38 and 39-and is connected to. the. .large. conduit: 34 by..means-- of thesmall conduit 48. -I'husitwill be seen .that the space 41andthespacefl.arainterconnected :and 41. isachieved through. the:operation of a double-disced valve which operates between the i-clowerdiaphragm assembly and the upper diaphragm assembly in response tomovements thereof. "The details of this valve are shown in Figure 2 inwhich the lower diaphragm Z4 and -.Zthe::upperrdiaphItagnf IS of thelower diaphragm ..assein=bly.contain a-va1ve receiving member 49. A

1 lowerrvalve member 50 is afiixed to the member 49 and the-seat Si'thereof is adapted to receive a' valve disc member in the form of a ball52. A

. stem 53 attaches the ball 52 to a disc 54 which is adapted to contactthe seat 55 of a valve mem ber 56 in a valve receiving member 5!attached tocthevdiaphragmszefli 3 H188 and 3,9,- the :upper diaphragm 1assembly. sA":.-re1ativelyaweak. zspring 58rnormal'lwurges. ztheidiscs-54 into engagement with the "seat 55; thereby closing.theaconduitsfi.i and Dreyentingapassagc.ofriair from the :space" 32upwardly through the port 60 and :space 40 and thence. tothe atmospheresthrough -port' 4i A conduit: "61 connectsthe': valve member 59 withthe-space -2 6.-between :theuiiaphragmsv 24 "and 25 and =willallowifreeiwpassageizof airt between the space-26 and .the space32-wheneverathe ball 52 is;raised from-the-.seat 5I. Thegspring 53engages an upper portion of thewal-vereceiv-i-ng:member 5.1m thaupperassembly.

At -onepoint on the :circumferencezof the vertical portion I 8., ofJ'the: guide or :piston I1 there is positioned. a .cam member- 62 which-:is fixed to: the portion 1 8 by means'of the dowel apin 63' and thescrew 64. The inclined surface -65 of thecamtz is adapted to be engagedbyionearm '66 of. alever =61...- the other arm 68? of which: is. inengagement with. a..-pin-.69 =whichsis part of valve :member 50 of the.lower diaphragm assembly. :The lever: 6! is pivotally'iconnected at tothe base of the diaphragm-containing!bodyaor casing 23. Th slot 1 I:makesit possible etc. change-the angle of the camsurface 65 .inresponse-to the. movement of the screw I3, thecam -body portion u -beingmade of a resilient material.

. .Anair supply sit-constant pressure is provided to the space I4between :the. piston I1 and the end portion -I 4.-.of.the;cylinder 20.'Ihe'valve I5 is a combination-pressure and= relieft regulatorwhichmawbesetso. as to 'add air to the space I4 upon outwardmovement of--the piston' II'or to. allow. airatoescapetthe. space 14 upon inwardmovement. otthe piston, *at all times "maintaining'athe pressurein'-;the1;;space I4 at a normally constant value.':A..diagrannnaticallyeillustrated':variablerestriction' -18.-is.employed::in ordertclimit free passage of air between the conduit34.andthe2space 41. The. disc II-is.-adapted=to.seatintothe conduit 34in .order to regulatetheficw-offain-therethrough. ThepositionI-of the-'-yalve.JIS-isadjusted by the set-screw-=I8..in. thee-body. 23 :andisprotected by means ofthe screw I9.

..In.theoperationofithe positioner it may be assumed'that a specificpresSureeXistsin chamber 35 ;..-that the-forces. applied to .thepositioix .are:. in equilibrium and the piston is stationary at. apositionawithinethe limitslofeits stroke. :A reduction of, pressur(chamber; 35 -Wi11Itth6IIT allow: the spring 42 to force thediaphragmsfill; 38,- 3i: and .30.;downwardly. and v.likewiser; move:the. doubledisced valve downwardly; In normal operation the spring 58will force the disc 54 into contact with the seat 55, and through thestem 53 will force the ball 52 into contact with the seat 5|, therebyclosing the conduit 6| and preventing passage of air from the chamber 26to the chamber 32. As the upper diaphragm assembly continues to movedownwardly in response to the urging of the spring 42, the disc 54 willbe lifted from the seat 55 to open the conduit 59 and allow the passageof air from the chamber 32 through the port 50 to the chamber 40 andoutwardly to the atmosphere through the port 4|. Since the chamber 32 isinterconnected with the chambers 29 and 41, the pressure above thepiston I! will be reduced to a valu lower than the pressure in thechamber 14 which is maintained at a constant pressure. The piston willthen tend to move upwardly, carrying with it the cam 62. The lever 61will then be rotated around the pivot and the lower diaphragm assemblywill tend to be depressed downwardly by th action of the spring 8|,keeping pin 59 in contact with arm 68 of lever 61.

As the lower diaphragm assembly is depressed downwardly, it carries withit the lower valve seat 5|. This action allows disc 54 to engage seat 55and close port 59 under the urging of spring 58. A further downwardmotion of the lower diaphragm assembly disengages the seat 5| from theball 52 and allows the passage of air under pressure through the conduit6| into the chamber 32 and through the conduits 33 and 34 into thechambers 29 and 41. When the force developed by the pressure in chamber29 exceeds the combined resistance to motion of the piston and stem,then the piston i! will be forced downwardly, carrying with it cam 62which in turn rotates lever 61 about pivot 10, thereby urging the lowerdiaphragm assembly upwardly through pin 69 which is at all times incontact with lever arm 68. This cycle is repeated until the forcesacting on both sides of the divider it are in balance and piston I! isin equilibrium at a position in exact accordance with that required bythe pressure in chamber 35.

If, however, the pressure within the chamber 35 is increased, such as byth addition of air under pressure through the port 35 from an outsidesource, such as the output of a pneumatic control apparatus, theequilibrium will be disturbed and the valve positioning apparatus willact to reach a new equilibrium. As the pressure within the chamber 35 isincreased, the upper diaphragm assembly will be forced upwardly againstthe force of the spring 42 and carrying with it the valve seat 55 andthe valve disc 54, thereby closing passage 59. The motion of the disc 54will be transmitted to the ball 52 through the stem 53 and will lift theball from its seat 5|. Air under regulated pressure will then passthrough the conduit 6| from the chamber 26 into the chamber 32 and willincrease the pressure Within the chamber 29 to a value in excess of thecombined resistance of the piston and stem, thereby forcing the pistondownwardly. Here again, as the piston descends, the action of the cam62, the lever 61 and the pin 59 will take place to force the lowerdiaphragm assembly upward and cause a reduction in opening between theseat 5| and the ball 52, thereby throttling the flow of air between thechambers 26 and 32. If the piston should descend too far, the seat 5|will continue to rise, engage ball 52 and unseat disc 54 whereupon airwill pass from the chamber 32 through the conduits 59 and 60 into thechamber 40 from which it exhausts to the atmosphere through the,

This dissipation of air pressure will- =port 4|. soon reduce thepressure in the chambers 32 and 29 and the piston will rise, withcorresponding lowering of the seat 5|, the ball 52 and the disc 54. Thedisc 54 will re-engage the seat 55 and stop the escape of air to theatmosphere. The cycle is repeated until a new equilibrium position hasbeen-reached by the piston, this position again being a function of thepressure within the space 35 in the upper assembly. So long as there isa deviation in the valve position from that position exactlycorresponding to the pressure applied in chamber 35, there Will exist animbalance of forces which will increase until the piston moves toeliminate the deviation.

The upper portion of the upper diaphragm assembly comprising thediaphragms 33 and 39 and the air chamber 4? are provided to compensatefor the tendency of the upper diaphragm assembly to move in response toa variation of air pressure in chamber 32. When the pressure in chamber35 increases to raise ball 52 from seat 5|, air under pressure flowsfrom the chamber 26 to the chamber 32 and upwardly through the conduit34 to the chamber. The eifective area of diaphragm 38 is greater thanthe efiective area of diaphragm 39 by an amount equal to the slicetivearea of diaphragm 39. Therefore the upward force developed on theunderside of diaphragm 39 by a change of pressure in chamber 32 isbalanced by an equal and opposite force downwardly in chamber 41 and theupper diaphragm assembly remains responsive only to a chang of pressurein chamber 35.

In practice a slight flow of air is allowed to escape past the valvedisc 54 through the small groove 55a. when the disc is seated. Thiscauses the ball 52 to remain free from the seat 5| by the slightclearance required to replac air escaping from the chamber 32 to theatmosphere when the system i in equilibrium.

It will b understood that when there is no restriction to flow of airfrom chamber 32 to chamber 41 through conduit 34, the stroking of theupper diaphragm assembly and therewith the ball 52 is dependent only andis directly related to a change of pressure in chamber 35. Byrestricting the flow of air from chamber 32 todl, the change in pressurein chamber 4'! lags behind the change of pressure in chamber 32, therebycausing the upper diaphragm assembly with ball 52 temporarily to movefarther than that required by the pressure change in 35, until theforces in chamber 32 and 3? are equal and opposite at which time theball 52 will again be restored to a position in correspondence with thepressure in chamber 35. The above action results in a more rapid changein the position of the piston than the rate of change of position of thepiston which would result from a change of pressure in chamber 35, ifthere were no restriction to flow in conduit 34. The variablerestriction in conduit 34 therefore provides a means of altering therate of response of the piston to a change in input control pressure.

The spring rate of the spring 32 establishes the range of pressure towhich the unit is responsive, and the limits of this range are determined by adjustment of the screw 35. It will be apparent that the riseofthe cam 52 bears a definite relation to the spring rate of spring 62.Because of the difiiculty in winding commercial coil springs to aspecified .spring rate, we have found it possible to adjust the rise ofthe cam in aeamoo order- :20 .coirrpensate' .fon: deviations: fromspecifiedspring rate' of thespring 42o As shown in'figme 1 the baseflthee'cam= Wis rastenedto thepiston' ii. "The: outer portion 120i thecam may beurged away from the bodyof' the cam 32 at bohemian-means ofadjustments of the screw 3. 13 adjusting the screw inwardlythe rise ofthe cam along the surface -5 can be reducedto a valuecorerspondingto thespring rate of thespring 42. 1

The embodiment of the invention illustrated inFi-gure 4, shows the:appiication of the positienerto a spring :ret-urndiaphragm motor valveof conventionaltype. In Figure 4- the diaphragm assemblies in the-body23 are the same asinl i-g re 1 operate 111 the-same way. Hence, it-

wi-ll not. be necessary to describ them in detail at thi time. Thepressure in the dome fit will be of a value to balance the thrust of thecompression spring 83 at any position in the stroke of the valve stem 8when the system is at equilibri-urn. It will be observed that a guide 8%is adapted to reciprocate Within the sleeve 3'5 which inturnis'afifb'zed to the bonnet E-a or" the diaphragm motor valve. -A divider83 corresponds generally to-the divider i iin Figure 1 except that ithas an opening 3B therethroughwhich. connects the chamber 23 with thedome'82. The effect of this arrangement is to allow free communicationof the pressure from the chamber 32 in the valve-pcsitioner with thechamber 29 and thedome 82. The spring 89 serves to maintain the'gu-ideat through the ball'fiil in continual contact with the stem 8 throughoutthe complete stroke thereof. A diaphragm 9i ofthe conventional resilienttype is provided and is secured between the bonnet 85 and the lowercasing 92 of the diapltiragm motor valve. A conventional disc isattached to the stem 8 and is movable therealong the axis of the stem.In Figure 4 a somewhat different arrangement of the adjustable camelement is shown along with a means for adjusting the cam rise to fitthe spring rate of the spring 42. The cam 9 3 is pivoted at 95 on aprojection 96 of the guide 34. The attitude of the face of the cam E imay be adjusted by means of the screws 13 by advancing or retracting thescrew in its threaded openin The rider 8? of the iever 6! is maintainedin contact with the face of the cam 94 by the thrust of the spring 3itransmitted mechanically through the lower diaphragm assembly, the pin59 and the arm 5% of the lever 81. The same effect may obtained bystressing the lower diaphragm assembly upwardly when assembling theapparatus and then allowing the air pressure to overcome the stress toposition pin 69 firmly against lever $8. The lever 5'5 is rotatablearound'the pivot t. Bydrivingthe, screw 13 inwardly it is posible tochange the slope of the cam face 9 with :espect to the longitudinal axisof the valve stem In this manner-themagnitude of restoration of thevalve seat" 5! is lessened relative to the stroke of the valve stem .8.A removable screw 93 provided in the bonnet 8B inorder to provide accessto the screw F3 for adjustment thereof.

-A different embodiment of the cam adjusting mechanism of the inventionis shown in Figure 5. The mechanism to which this embodiment is appliedis generallysimilar tothat illustrated in Figure 1 in whicha casing isattached-to the. positioner body. 23 and has a-window or-slit I9 thereinto receive thehead of an adjusting screw '13. Here againthehead of thescrew 73 serves as anindex toindicate the position-of the valve 8, disc;Apaeking-ring' provided totmaihtam fluid tight engagement: between the;piston 11. and;- the casing. 20. Alerter; 9.9 is pivotedat me on aprojectionof the bodyazi. One, arm llllcoi' theleverIQS engages the pin.69: which is a. partof the valvemember; Sill-.01 the. lower diaphragmassembly. Another arm .ltl-Zuof. the; lever. 89 "is adapted toengage.the, cam member 1&3 through the rollerorrider 1M. Thegcamj I63. ispivoted at 05 to t'he'piston- IT. The cam 1031 is freeto move atuitslower end in response to.;movementof the screw #3. Thesroller I88. isadapted: to move in the channel me which is. dish-shaped in ordertoallow for slight rotative-movementof the piston l'T-withint-hecasing20.

It is thought that the inventifon and its nu! merous attendant:advantages will be fuily under.- stood from the foregoingdescriptiomand' it is obvious that numerous changes maybe madeinthe-"form, constructionand-arrangement of the severalpartswithoutdeparting fromthespirit or scope of theinvention; or sacrificing any ofits attendant-advantages,the form herein disclosed being a preferredembodiment for the purpose of illustrating the invention. Itwillbeunderstood that the invention-is applicable to the positioningofdevices other than valves in response to the output of a fluid pressurecontroller.

' The invention is hereby'claimed as follows:

1; A-valvepositioningdevlce operable in respcnseto the-output ofafiuidpres-sure controller comprising a casing, 'fluid pressure operated;mean-s operatively associated with a member to be positionedguide meansassceiatedwithsaid means and movable axially-within said casing, meanson one side ofsaid' firstmeansadapted to-continually urge saidfirst-means in one direction; a plural" diaphragm means for supplyingpressure to the opposite side ofsaid first means in response to theoutput of a'fiuid pressure, c011 trolley including a-source .of fluid,pressure, a lower diaphragrn'assembly and an upper diaphragm'assembly,valve seats in each assembly. a double .disced valve. adapted to seat insaid valve seats-and a main spring normally urging sai" upper assemblytoward said lower assembly, and cam means operatively associated withsaid member adapted to controlthe positioning of said lower assemblyvalve. seat in response to movement of said member.

2. A valve positioning device operable re,- nonse to. the output of afluid pr ssure controller pri ng. a a ing, fluid pre sure operated meansop ively asso iatedwith ameinber to be positioned, guide meansassociatedwith said means and movable axially within said casin meansoneneside ofesaid first means adapted to continually urgesaid firstmeans in one. direction, a plural diaphragm means. for supplyingpressure tothe. opp site side. of. said first m ans 1. r spon e t theoutputof afluid pressure om trailer including a source of fluid pressura l w r d aphragm. ass mbly. and an upper diaphra m assembly, valveseats. in each assembly, a double-disced valveadaptedto seat in saidvalve seats and a .main' spring normally .urging said upperassemblvtoward said lower assembly, and meansadaptcdto control thepositionin of said. lower assembly valve .seatlin response to movement.of. said member including adjustable. cam element operatively associatedwith. I sai member. and. aalever oivotallv mounted for en easement withsaidcamand said seat.

a 3 A valve positioning device. operabl in responsetmthe output 03 afluid pressure-controller comprising a casing, fluid pressure operatedmeans operatively associated with a member to be positioned, guide meansassociated with said means and movable axially within said casing, meanson one side of said first means adapted to continually urge said firstmeans in one direction, a plural diaphragm means for supplying pressureto the opposite side of said first means in response to the output of afluid pressure controller including a source of fluid pressure, a lowerdiaphragm assembly and an upper diaphragm assembly, valve seats in eachassembly, a double-disced valve adapted to seat in said valve seats anda main spring normally urging said upper assembly toward said lowerassembly, and means adapted to control the positioning of said lowerassembly valve seat in response to movement of said member including anadjustable cam element operatively associated with said member, a leverpivotally mounted for engagement with said cam and said seat and meansfor varying the pitch of said cam member to match the spring rate ofsaid main spring.

4. A valve positioning device operable in response to the output of afluid pressure controller comprising a casing, fluid pressure operatedmeans operatively associated with a member to be positioned, guide meansassociated with said means and movable axially within said casing, meanson one side of said first means adapted to continually urge said firstmeans in one direction, a plural diaphragm means for supplying pressureto the opposite side of said first means 10 in response to the output ofa fluid pressure controller including a source of fluid pressure, alower diaphragm assembly and an upper diaphragm assembly, valve seats ineach assembly, a double-dissed valve adapted to seat in said valve seatsand a main spring normally urging said upper assembly toward said lowerassembly, and cam means operatively associated with said member adaptedto control the positioning of said lower assembly valve seat in responseto movement of said member, a portion of said upper diaphragm assemblyadapted to resist movement of said assembly in one direction in responseto fluid pressure communicated thereto and adjustable restricting meansassociated with said portion for controlling the rate of application ofsaid resistance.

JAMES P. HENDERSON. JOHN C. GALL.

References Cited in the file of this patent UNITED STATES PATENTS NumberName Date 2,044,936 Donaldson, Jr June 23, 1936 2,098,914 Gorrie Nov. 9,1937 2,291,048 Lichtenstein July 28, 1942 2,382,941 Moore Aug. 14, 19452,411,748 Kelley Nov. 26, 1946 2,536,184 Johnson Jan. 2, 1951 2,558,506Annin June 26, 1951 2,612,870 Roetter Oct. 7, 1952

